Abstract
Time-resolved photoelectron spectroscopy is commonly employed with the intention to monitor electronic excited-state dynamics occurring in a neutral molecule. With the help of theory, we show that when excited-state processes occur on similar time scales the different relaxation pathways are completely obscured in the total photoionization signal recorded in the experiment. Using non-adiabatic molecular dynamics and Dyson norms, we calculate the photoionization signal of cytosine and disentangle the transient contributions originating from the different deactivation pathways of its tautomers. In the simulations, the total signal from the relevant keto and enol tautomers can be decomposed into contributions either from the neutral electronic state populations or from the distinct mechanistic pathways across the multiple potential surfaces. The lifetimes corresponding to these contributions cannot be extracted from the experiment, thereby illustrating that new experimental setups are necessary to unravel the intricate non-adiabatic pathways occurring in polyatomic molecules after irradiation by light.
Highlights
Ab initio calculations as well as non-adiabatic dynamics simulations can help in the endeavour of identifying the states involved in the ionization and following the vibrational dynamics on the states of interest
On the way to simulate the TRPES of cytosine, we computed first the ground state photoelectron spectra
Cytosine is an excellent example to demonstrate that single-photon time-resolved photoionization alone is not sufficient to identify the hidden excited-state deactivation dynamics in the neutral molecule
Summary
Ab initio calculations as well as non-adiabatic dynamics simulations can help in the endeavour of identifying the states involved in the ionization and following the vibrational dynamics on the states of interest. Only the neutral excited-state dynamics is simulated and compared to pump-probe photoelectron yields, which reflect the neutral dynamics only indirectly This simplified approach relies on the assumption that high-lying states give a full ionization signal while no signal is obtained if the population has proceeded to low-lying states[22]. Kotur et al.[36] combined time- and mass-resolved photoionization with ab initio electronic structure theory in order to track the time constants of some selected excited-state dynamics pathways of the neutral molecule While these experiments use strong-field multiphoton ionization as a probe, Ullrich et al.[31] employed a 200 nm probe laser, which is capable of ionizing cytosine with a single photon from the excited states. We show that the observed photoelectron signal does not reflect the underlying processes from the dynamics of the neutral molecules, pointing to a very complex excited-state dynamics which cannot be disentangled alone by single-photon ionization experiments
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